ESTRO 2021 Abstract Book

S556

ESTRO 2021

SP-0721 Patient positioning and immobilisation for advanced radiation therapy L. Mullaney 1 1 Trinity College Dublin, Applied Radiation Therapy Trinity Research Group, Discipline of Radiation Therapy, School of Medicine, Dublin, Ireland Abstract Text Reproducibility is an essential component of radiation therapy treatment since its inception. The ability to accurately reproduce the internal and external anatomy of the patient allows for exploitation of the therapeutic ratio. Patient positioning and immobilisation ensures the reproducibility of the treatment site from planning to treatment, allowing for accurate delivery of dose to the target volume, minimising dose to the organs at risk, and as result, improving outcome. It could be argued that in the era of advanced imaging and adaptive treatment, immobilisation is outdated. However, the starting point for all advanced radiation therapy techniques is the assumption that the patient is accurately positioned and immobilised to ensure inter- and intra-fraction motion is kept to an absolute minimum. Based on this assumption, opportunities, such as dose escalation and altered fractionation scheduled, can be perused. RTTs have a vital role in ensuring the optimisation of patient positioning and immobilisation in all radiation therapy scenarios. Abstract Text The introduction of MR-guided radiotherapy on a MR-linac has provided the potential to improve treatment by daily online adaptation of the treatment plan to the patient’s anatomy. The superior soft tissue contrast of the MR makes it possible to accurately delineate the GTV and the organs at risk during each treatment session. Therefore, it is possible to use smaller margins, which result in a higher dose to the target and a lower dose to the organs at risk. However, a disadvantage of online adaptive radiotherapy is the increased treatment time: (extra) time for MR acquisition, registration, adjusting the delineation, re-planning and the irradiation itself, which can take a total of 45-60 minutes. During this time the patient remains on the treatment couch. The position of the patient needs to be stable and comfortable, and the patient should be able to endure this possible lengthy procedure. Due to the limited bore opening of MR-linacs, it is sometimes difficult to position the patient depending on the treatment site and the posture of the patient. What are the patient positioning possibilities for a MR-linac? It is easy to copy the position protocol from a conventional linac, but is this the best way to go? This presentation will focus on these questions and provide explanations and some solutions. SP-0723 Immobilisation for proton in abdomen J. Thiele 1 1 Strahlentherapie und Radioonkologie Universitätsklinikum Dresden / OncoRay Dresden, Protonentherapie, Dresden, Germany Julia Thiele: Immobilisation for proton beam therapy of tumours in the abdomen Julia Thiele 1 , Sergej Schneider 2,3 , Chiara Valentini 1,2 , Fabian Lohaus 1,2 , Danilo Haak 1 , Mechthild Krause 1-5 , Christian Richter 1-3 , Esther G.C. Troost 1-5 . 1 Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany 2 OncoRay - National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Helmholtz-Zentrum Dresden – Rossendorf, Dresden, Germany 3 Institute of Radiooncology - OncoRay, Helmholtz-Zentrum Dresden-Rossendorf, Rossendorf, Germany 4 German Cancer Consortium (DKTK), Partner Site Dresden, and German Cancer Research Center (DKFZ), Heidelberg, Germany 5 National Center for Tumor Diseases (NCT), Partner Site Dresden, Germany: German Cancer Research Center (DKFZ), Heidelberg, Germany; Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany, and; Helmholtz Association / Helmholtz-Zentrum Dresden - Rossendorf (HZDR), Dresden, Germany; Abstract : Due to the physical characteristics of protons, proton beam therapy (PTx) differs from photon treatment, in terms of radiation field arrangement, dose distribution, and uncertainty in dose deposition. Therefore, although the immobilization procedure is similar, some peculiarities must be taken into account, since even the image guided radiotherapy (IGRT) methods are also differently applied, with e.g., bony setup being mandatory for PTx. In the University Proton Therapy Dresden, selected patients with tumours of the upper abdomen are treated with proton therapy. Before simulation, three (gold) fiducial markers are implanted in the proximity of the tumour. Abdominal corsets [Schneider] or compression devices to reduce respiratory motion in the target volume are used for computer tomography (CT)-based treatment planning and daily PTx application. Moreover, the planning-CT is performed as a 4D-CT; target volumes, organs at risk and the fiducial markers are contoured on each of the eight breathing phases and on the average CT. The PTx planning needs to be robust in order to take movement of target structures and organs at risk into account. In general, a pencil beam scanning technique (PBS) is applied. SP-0722 Online adaptive radiotherapy - Is traditional immobilisation necessary? S. Conijn 1 1 The Netherlands Cancer Institute, Radiotherapy, Amsterdam, The Netherlands